Portable electronic devices having integrated hard disks are being increasingly marketed in the entertainment industry. Devices of this type may be jarred or dropped during use, and the associated movement of the write/read heads may result in damage to the memory disks during operation and therefore loss of data.
To avoid damage of this type, three-axis acceleration sensors are typically used which are able to identify a free fall, and thus make it possible to move the write/read head into a parked position in a timely manner. A free fall is identified when an almost zero acceleration is measured on all three axes. However, detection of a free fall is problematic when the hard disk rotates about the center of gravity of the device and the sensor is situated outside the center of gravity. For a motion of this type, the acceleration sensor is able to measure a centrifugal acceleration, in which case the free fall cannot be identified. Complicated signal processing is necessary to also identify this motion. However, other motions are also possible which prevent a determination of the free fall as a function of the signal of an acceleration sensor. Thus, for a rotational speed ω and a distance r of the sensor from the center of gravity of the hard disk or of the device in which the hard disk is installed, the centrifugal acceleration a=r*ω2 would reach gravitational acceleration g=9.81 m/s2 and therefore simulate a normal state. A signal of this type could be generated, for example, by providing a sensor at a distance of 2.8 cm from the center of gravity, the device rotating at 3 revolutions per second during the free fall. These parameters represent typical values for a falling MP3 player or cell phone.
In addition, the center of gravity of some devices changes during use. Thus, some mobile devices are equipped with folding or sliding mechanisms which make it impossible to position a permanently installed sensor unambiguously in the center of gravity. The same applies for devices whose center of gravity is situated outside the housing, which is the case for an opened laptop, for example.